Terminate with error if missing timeseries file path

[wpbonelli]

When a UZT input file is using timeseries but missing the 'TS6 FILEIN ' line the program can crash. Maybe this also affects other packages, not sure.

Discovered by @Manangka in https://github.com/modflowpy/flopy/issues/2280#issuecomment-2278142136

When timeseries are defined a hashtable is created storing the timeseries data. In this case that wasn't happening because of the missing line. There is a dependency on the timeseries however in the perioddata of the uzt package. When it tries to retrieve the value of PFOS it results in a crash. We should probably create an issue to prevent a hard crash. A simple null check could be sufficient